I figured it out. They added cesium to get rid of the effect of the ejector acting as a large corner reflector and blowing up the rear-hemisphere RCS. Ionizing the exhaust plume made it act like a solid tail fairing, still not spectacular but far less reflective than a pure corner reflector. This was part of the progression from A-11 (Lockheed's "you guys are stupid there's no way to reduce the RCS so we're ignoring that part" design) to A-12 (Lockheed's "well damn, you really can reduce the RCS" design). This is also why you find large fairings covering the engine nozzles in images of a lot of the RCS pole models.
If it happened to affect IR sensors, that was an added benefit and was not the point of the effort. Kinda like how the chines were actually added to reduce side-on RCS, not for aerodynamic purposes, but they had a beneficial aerodynamic effect on the design (despite what some nutcases want you to believe).
At the end of the day they did manage to get the RCS down, but the appearance of TALL KING and its new capabilities basically obliterated most of their work which was designed to overcome current-generation (at the time) blip-scan radar sets. And on more than a few flights over Vietnam or near the DPRK, there was no evidence whatsoever that anybody knew the jet was around. Of course, on other occasions, they were tracked or even fired at, so it clearly wasn't a VLO platform by any stretch of the imagination. But it did represent the first operational aircraft designed to have a reduced RCS, and a lot of the ideas did work to greater or lesser degrees. The best read on the subject is From RAINBOW to GUSTO by Paul Suhler, which is basically a history of the A-12 program from an RCS reduction standpoint.